It's currently autumn of 532. My last migrant wave was summer of 517 - 15 years ago.
In that time, plenty of additional digging has occurred, fortress wealth has increased dramatically (created wealth currently stands at 22 million, exported wealth at 35 million). Almost all the living and working space has been smoothed and engraved, although the mining areas are unimproved.
All three caverns have been beaten into submission, and the magma sea and magma pipe have been tapped for a beyond healthy metal/glass/ceramic industry (20 elite marksdwarves training year-round can't use enough bolts to prevent the imported goblinite from expanding my metal stores). 30 mostly legendary melee troups routinely destroy 100+ goblin sieges and 200+ undead rage parties (although it seems we've offed all the local necros, the undead swarms still show up on occasion), usually without even chipping a nail, unless someone experiences a "dwarven moment" and dodges off the edge of the 10-z cliff into the river or something (seriously, my 400+ kill champion just did that last siege... grrrr...).
Caravans and liaisons/diplomats usually arrive and depart safely (although there have been occasional mishaps through the years, those tend to be the exceptions), caravans usually with a >1 million profit (due largely to the ridiculous value of masterwork meals).
Have dug out the safe parts of 3 separate candy spires, resulting in ~1500 units, 75% of which have been processed at this point (but not yet used for armor/weapons/whatever, as, honestly, the steel-clad military is badass enough without it).
A significant portion of my paved roads have been upgraded from various stone types to platinum, gold or silver; the trade depot is currently made out of platinum. Many artifacts have been created, and most of the ones that are usable have been built in appropriate places (I think I've got two extra artifact floodgates still in storage, though). Royal communal dining room is ridiculous - entirely engraved by masters, 30-ish platinum chairs and tables, slowly being upgraded to masterwork as I import more platinum, as I think I've pretty much exhausted the local supply.
95% of the population, including the count and the mayor are beyond ecstatic. A couple of the military are simply fine, but they're definitely in the minority.
Very few deaths in recent years, mostly due to old age, but a couple military losses here and there, so not a "death trap" situation; although we are now down to 150 from 180+, most of those deaths were more than 10 years ago - the last 10 years have generally been very safe.
Have verified through legends mode that the civ (and others) are still alive and healthy.
The only two things I can think of that haven't been pursued yet are breaching HFS and attracting the monarch (which I've put off because I kinda like being able to tell the caravan what I want...). I haven't read anywhere that migration is tied to either of those things, other than possibly breaching HFS being a detriment, which doesn't seem to be completely validated.
What else might I be missing? Is it possible that the couple of particularly ugly sieges that resulted in 10-15 deaths each, but occurred between 15-20 years ago are still haunting me? How long do the "death trap" and/or "despite the danger" type perceptions of a fortress last? Does migration stop if you don't work on pulling in the king/queen? Has my civ just decided that we've been successful enough we don' need no stinkin' migrants?